Digital modulation methods include pure phase modulation methods in which the signal phase angle carries the digital information and the signal amplitude is not significant, amplitude modulation methods in which the information is carried by varying amplitude levels, and hybrid methods. Even using pure phase modulation methods, when a signal passes through a channel suffering from impairments such as echoes or time dispersion, the received signal will have amplitude variations that depend on the underlying information bits. Special detection methods which have good performance characteristics in these circumstances make use of the information in the signal amplitude variations. Whenever information is carried by amplitude variations, some scaling is necessary at the receiver to remove the arbitrary amplitude change between transmitter and receiver over the arbitrary propagation path. This scaling is effected by automatic gain control (AGC) in the receiver. A dilemma arises in deciding whether an AGC system should adapt rapidly or slowly to perceived changes in the received signal; if too fast, the information-carrying amplitude variations may be partly removed; if too slow, the receiver will fail to adapt to changing propagation path variations, due, for example, to a mobile receiver changing position.
The archetypal AGC system which has been used for approximately 60 years in analog, AM receivers are based on the information modulation usually swinging the amplitude of the transmitted signal equally above and below a mean value. By detecting the mean value of the received signal with the aid of a moving average calculation, i.e., a low-pass filter, it can be decided if the mean received signal level at the detector is in the desired range, and if not, the gain of some amplifying stage or stages in the receiver is increased or decreased to bring the detected mean within the desired range.
In addition, prior art systems exist concerning the selection of optimum time constants, thresholds, dead bands or distribution of gain control between different amplifying stages. Lately, the emergence of devices for numerical signal processing have given more flexibility to the designer to implement optimum AGC strategies within a microprocessor program.
With the development of radar, it was apparent that known AGC methods were unworkable, since it was impossible to predict the strength of an echo return from a target in advance in order to decide the appropriate receiver gain. Instead, a type of receiver and detector known as a logarithmic amplifier was devised. This type of receiver consists of a chain of progressively saturating (limiting) amplifier and detector stages, wherein the detector outputs are summed. Weak signals are only able to operate the detector at the end of the amplifying chain. As the signal level increases, this last amplifier and detector saturates, while the preceding stage begins to contribute to the output, and so forth. Thus, the device gives a unit increment in the summed detector output signal every time the input signal increases by a factor equal to the amplification per stage, hence the logarithmic characteristic. Such a receiver thus circumvents the need to employ AGC in order to function over a wide dynamic range of signal inputs.
Another distinct AGC principle used in frequency-hopping receivers is known as memory AGC. When a receiver cycles systematically or pseudorandomly among a number of frequency channels under control of a frequency-hop processor, different propagation losses on different frequencies can require that the gain be controlled according to the frequency selected. This may be done by use of digitally gain-programmed amplifier stages, wherein the gain setting for each selected frequency is recalled from a memory. After receiving a "hop" or burst of a signal on a selected frequency, the gain setting is updated and written back to the memory against that frequency so that the updated value will be used next time. Strategies for partially updating the gains that will be used on other frequencies from observations made on one frequency can sometimes be devised to ensure that the gain can adapt sufficiently fast even when each channel out of a large number of channels is infrequently selected.
A further distinct AGC principle related to radar logarithmic amplifier techniques is described in U.S. Pat. No. 5,048,059 entitled "Logpolar Signal Processing", which is incorporated herein by reference. A radio signal, having both a phase angle and an amplitude, requires a pair of number sequences to fully describe it. Conventionally, the Cartesian vector representation had been used, where the radio signal is described by an X (cosine or In phase component) and a Y (sine or Quadrature component). The radio signal would be resolved into its I and Q components by multiplying it with a cosine reference signal and a sine reference signal, smoothing the results and then digitizing them for subsequent numerical processing. In the conventional approach, AGC was required to hold the signal level at the point of digitizing within the optimum part of the dynamic range of the Analog to Digital convertor.
In the method described in the aforementioned patent, Cartesian representation was not used, but rather a polar representation in which the logarithm of the signal amplitude was determined by digitizing the detector output of a radar-type logarithmic receiver, simultaneously with digitizing the saturated output of the final amplifier stage to obtain a phase related value. In this way it was possibly to digitize a radio signal preserving its full, vector nature before determining an AGC scaling. The scaling to use for best demodulating a signal containing information in its amplitude variations can then be determined by post-processing in a numerical signal processor.
An issue closely related to AGC in receivers is automatic frequency control (AFC). The purpose of AFC is to remove frequency errors associated with transmitter or receiver frequency inaccuracies or Doppler shift due to relative movement which otherwise would hinder the extraction of information carded by frequency or phase modulation. A similar dilemma exists in the design of AFC systems as in the design of AGC systems, namely how to separate variations caused by the unknown underlying information that has to be determined from the other sources of variation. U.S. Pat. No. 5,136,616 describes a method whereby several postulates of an AFC control value are held in conjunction with corresponding postulates of the data modulation sequence underlying the received signal, the AFC values being retained and updated or discarded along with deciding which of the associated data sequences are most likely to be correct.